The present invention relates in general to the reduction or beneficiation of metallic ores and in particular to the reduction of iron bearing ores to a desired state.
Regardless of its end use, iron must be obtained from its ores, most often oxides such as hematite and magnetite, as the result of a reduction process. In the reduction process, oxygen chemically bound to the metal in the ore is chemically transferred to the reducing agent thus leaving the metal in its free metallic form. Typically, the reducing agent comprises carbon which may be in the form of charcoal, coke or carbon monoxide gas. Although various processes and apparatus have been used to facilitate the reduction or iron ore, the most prevalent have traditionally involved the use of blast furnaces. A blast furnace of the type suitable for reducing iron ore normally comprises a vertically disposed, cylindrical steel shell lined with a refractory material such as alumina. Weighted amounts of coke and iron ore and limestone are introduced at and descend from the top of the furnace while a continuous blast of hot air is blown into and ascends upwardly from the bottom of the furnace. As the coke and iron ore charge moves slowly downward, it rapidly increases in temperature enabling the oxides to be reduced, the coke serving to remove the oxygen from the iron ore. About two thirds of the way down the furnace, the iron melts and is deposited as molten iron in the furnace hearth. However, notwithstanding the long use of such conventional blast techniques and the numerous improvements therein, this method of reducing iron ore has proven relatively unattractive from both a pollution and safety viewpoint. Moreover, the expenses involved in operating blast furnaces further detract from their desirability.
The prior art is replete with examples of efforts on the part of industry to improve on the efficiency and effectiveness of the conventional coke oven-blast furnace process for reducing iron ore. For example, U.S. Pat. No. 3,031,293 to Meissner discloses a reducing process wherein a mixture of solid particles of iron oxide is discharged downwardly into a rising column of reducing gas so as to entrain the smaller particles within the gas until reduction is completed. In one embodiment of the disclosed apparatus, the iron oxide is introduced directly into a restricted Venturi portion of the duct through which the reducing gas is caused to flow. On the other hand, U.S. Pat. Nos. 2,287,476 and 2,365,194, both to Hodson et al, disclose iron ore reducing apparatus wherein injection nozzles are used to create a flow of reducing gases and powdered iron ore within a furnace shaft of sufficient turbulance to suspend the ore in the shaft until the ore is reduced to metal. In the former disclosure, separate nozzles are used to inject the gases and the ore, the nozzles being tangentially arranged about the periphery of the shaft, while in the latter patent an apparatus is disclosed utilizing a common nozzle to inject both the ore and reducing gas. Furthermore, U.S. Pat. No. 2,287,476 teaches the use of a furnace shaft having a converging frusto-conical upper portion for gradually increasing the upward velocity of the suspension. The use of a turbulent flow of reducing gas to promote the reduction of finely powdered iron ore by causing an intimate association therebetween is further illustrated by the process taught in U.S. Pat. No. 2,307,997 to Dill. None of the foregoing techniques, however, have gained much acceptance in the industry which still largely relies on the conventional blast furnace for producing much of its iron.
Relatively recently, attention has been directed to various fluidized bed direct reduction processes for producing metallic iron. In such a process, a reducing gas is introduced into a bed of finely divided ore particles under suitable pressure and velocity such that the ore particles become suspended in the gas rendering the bed of solid particles "fluid". U.S. Pat. No. 3,944,413 to Volk is exemplary of prior art fluidized bed reduction apparatus. While environmentally cleaner than the blast furnace approach, fluidized bed techniques suffer other drawbacks. In particular, the process is relatively expensive due primarily to the required reaction period of several hours to achieve ore reduction.
Another method and apparatus for reducing iron ore is taught in U.S. Pat. No. 3,485,487 to Bennett et al. According to the Bennett et al disclosure, iron ore is combined with a fluidizing gas such as butane in a fluidizing tank and subjected to a high velocity-pulsating flow of gases produced by explosions in the exhaust of a pulse jet engine. The explosions and resulting supersonic pressure waves uniformly heat the fluidized ore which is blown out the exhaust into a collecting chamber.
Numerous individual deficiencies characterizing the prior art iron ore reducing technologies are overcome by the present invention. In this regard, the present invention contemplates a method and apparatus for reducing metallic ores, in particular iron ores, which simultaneously offers the advantage of rapid reaction rates using relatively simple and inexpensive equipment resulting in a high tonnage output of metallic iron at reduced costs. Moreover, the process is relatively pollution free, highly energy conserving and characterized by a reduced degree of occupational hazards. The foregoing advantages are achieved by the present invention as the result of a reducing technique differing from the prior art in the manner of processing finely powdered iron ore with a reducing gas.